E.A. Bell, M.D. Bell, G. Morgan and L. Morgan Bell, E.A.; M.D. Bell, G. Morgan and L. Morgan. The recovery of seabird populations on Ramsey Island, Pembrokeshire, Wales, following the 1999/2000 rat eradication
The recovery of seabird populations on Ramsey Island, Pembrokeshire, Wales, following the 1999/2000 rat eradication
E.A. Bell1, M.D. Bell2, G. Morgan3 and L. Morgan3
1Wildlife Management International Ltd, PO Box 607, Blenheim 7201, New Zealand,
Abstract Ramsey Island, 259 ha, is ca. 1 km off the Pembrokeshire coast, south-west Wales. The eradication of brown rats (Rattus norvegicus) was successfully completed in the winter 1999/2000 using a ground-based bait station operation. The pre-eradication survey using tape playback estimated the Manx shearwater (Puffinus puffinus) population to be 849 pairs. These surveys were repeated in 2007, 2012 and 2016. Each survey showed the Manx shearwater population had increased, reaching 4,796 pairs in 2016 with birds spreading from previously known breeding locations. European storm petrels (Hydrobates pelagicus) were first recorded breeding on Ramsey Island in 2008 with up to 12 pairs in 2016 (a minimum estimate based on accessible survey areas). Other species have also shown improvements to population estimates and range since the rat eradication. This evidence shows that there can be little doubt that the presence of brown rats on Ramsey played a significant role in suppressing breeding numbers and limiting the breeding range of seabirds on the island and the positive results following the successful eradication are now being seen.
Keywords: brown rat, European storm petrel, Hydrobates pelagicus, Manx shearwater, Puffinus puffinus, Rattus norvegicus, survey
INTRODUCTION STUDY AREA AND METHODS Rats are known to have devastating effects on seabird Study site and land bird populations by predation of eggs, chicks Ramsey Island, 259 ha (5°20′W, 51°51′N), is located and adults which reduces breeding success, recruitment, about 1 km off the Pembrokeshire coast, south-west Wales population size and distribution. They have caused (Fig. 1). It is a nature reserve owned and managed by the extinctions of birds on numerous islands throughout the RSPB. Ramsey Island is approximately 3.2 km long and world (Moors & Atkinson, 1984, Atkinson, 1985, Towns, 1.6 km across at its widest point and is surrounded by et al., 2006; Jones, et al., 2008). Smaller burrowing seabirds coastal cliffs which are particularly high and steep on the are recognised as the species most affected by invasive rats western side of the island. There is also a number of small (Jones, et al., 2008; Towns, et al., 2011). The eradication of islets (including a chain of islets from the southern end) rats from seabird islands is recognised as one of the most and caves around the coastline. The coastline of the island important tools in avian conservation in recent times, with is made up of exposed rocky shores with a small number of significant long-term restoration benefits such as increased sandy coves. The top of the island is gently rolling and is productivity and populations sizes and establishment of dominated by two prominent peaks (Carn Ysgubor 101 m new, or return of previously locally extinct, seabird species and Carn Llundian 136 m). The island supports three main being achieved (Atkinson, 1985; Moors, et al.,1992; habitats; acid grassland, bracken-dominated grassland and Lock, 2006; Ratcliffe, et al., 2009; Booker & Price, coastal heathland (Doncaster, 1981; Hurford & Evans, 2010, Bourgeois, et al., 2013; Le Corre, et al., 2015).The 2006; CCW, 2008). The heathland and maritime grassland protection and enhancement of UK seabird breeding habitat communities are of conservation importance in Wales has been recognised as an important conservation priority, (JNCC, 2001; Hurdford & Evans, 2006; CCW, 2008). including under international conservation agreements The rush-pasture fields are grazed by rabbits Oryctolgus( (Brooke, et al., 2007; Ratcliffe, et al., 2009; Dawson, et cuniculus), ponies (Equus caballus) and sheep (Ovis aries) al., 2015; Thomas, et al., 2017). Over 400 islands around as part of the management to support wildlife, particularly the world have been successfully cleared of rats, including choughs (Pyrrhocorax pyrrhocorax) (Doncaster, 1981; twelve in the United Kingdom, with a subsequent increase Long, 2003). The bank vole (Myodes glareolus) and in bird populations (Thomas & Taylor, 2002; Towns & common shrew (Sorex araneus) are also present on the Broome, 2003; Jones, et al., 2008; Howald, et al., 2007, island. DIISE, 2015, Thomas, et al., 2017). A feasibility study of eradicating brown rats (Rattus norvegicus) from Ramsey Island was completed in 1998 and led to the ground-based eradication in autumn 1999. Documenting the recovery of bird species on islands that have had invasive mammals removed is becoming increasingly important. RSPB has been monitoring bird populations on Ramsey Island since 1992. Due to difficulty in accessing natural burrows, between 2013 and 2016, RSPB constructed a man-made seabird habitat using artificial burrows with the aim to establish a Manx shearwater (Puffinus puffinus) colony that could be used to monitor productivity, recruitment and adult survival. This paper details the changes to the Manx shearwater population on Ramsey Island, including within the man- made habitat, and the subsequent colonisation of the island by European storm petrel (Hydrobates pelagicus) following the eradication of brown rats. Fig. 1 Location of Ramsey Island, Wales.
In: C.R. Veitch, M.N. Clout, A.R. Martin, J.C. Russell and C.J. West (eds.) (2019). Island invasives: scaling 539 up to meet the challenge, pp. 539–544. Occasional Paper SSC no. 62. Gland, Switzerland: IUCN. Island invasives: scaling up to meet the challenge. Ch 3C Strategy: Outcomes
The island is part of the Pembrokeshire Coast National of the island on hills Carn Llundain and Carn Ysgubor Park and has a range of designations including as a Site of (I.D. Bullock, unpublished data; Perkins, et al., 2017). For Special Scientific Interest (SSSI), National Nature Reserve burrow counting and sampling purposes, Ramsey Island (NNR), Important Bird Area (IBA), Special Protection was divided into 42 sub-areas by topographical features. Area (SPA) and Marine Special Area of Conservation A full count of suitable burrows (i.e. more than 0.7 m in (MSAC) (JNCC, 2001; CCW, 2008; Hayhow, et al., length and not doubling back to the surface) in these 42 2016). Ramsey holds important breeding populations of sub-areas was completed in 1999, 2007, 2012 and 2016. razorbill (Alca torda), guillemot (Uria aalge), kittiwake (Rissa tridactyla), Manx shearwater, chough and wheatear Estimation of the numbers of Manx shearwaters (Oenanthe oenanthe) (JNCC, 2001; Johnstone, et al., on Ramsey Island was based on playback of recorded 2011). Ramsey Island is also an important breeding site for calls (Brooke, 1978; Smith, et al, 2001; Perkins, et al., Atlantic grey seals (Halichoerus grypus). 2017). This method relies on the fact that if a male Manx shearwater call is played down a sample of burrows The island is popular with visitors who are interested during the incubation period, most incubating males, but in the seabirds, land birds, flora and history. These visitors no incubating females will respond to that call (Smith, et travel to the island on small passenger vessels from St al., 2001). For a given number of breeding pairs, it is then David’s between April and October. There is a jetty and possible to establish the number of males that respond to several buildings on the island, including the warden’s recorded calls. From this, using the following formula it home and information shelter. is possible to estimate the number of breeding pairs in the burrows on the island. Manx shearwaters have been recorded on Ramsey Island since the 18th century (Mathew, 1894; Holloway, 2010; Lovegrove, et al., 2010). Historical reports and more recent seabird monitoring on Ramsey recorded declines The response rate for Manx shearwaters was calculated in Manx shearwaters and other seabirds (Mathew, 1894; by Bullock in 1999 (0.409) and was based on a study set Humpridge & Bullock, 1999; Lovegrove, et al., 2010). of 13 burrows (Humpridge & Bullock 1999). Alternative These declines were attributed in part to the presence of response rates were available from Skomer (0.43, Smith, brown rats and predation on eggs and chicks (Humpridge et al., 2001) and Skokholm (0.505, Brooke, 1978) or the & Bullock, 1999; Lovegrove, et al., 2010). seabird monitoring handbook (0.505, Walsh, et al., 1995). The Ramsey response rate of 0.409 was used in 2007 as It is not known when brown rats became established on it allows direct comparison to the earlier survey on the Ramsey Island; but this was likely to have occurred more island. The response rates were recalculated for the 2012 than two hundred years ago from an early shipwreck. survey (to 0.4625) using methods developed by Murray, et al. (2003), Newton, et al. (2004) and Perrins, et al. (2012). Brown rat eradication The response rates were recalculated for the 2016 survey (to 0.845) which used dual-sex calls which had been shown The eradication of brown rats was completed as a to give a more reliable correction factor (Perkins, et al., ground-based operation using rodenticide cereal blocks in 2017). protective bait stations to reduce risk to non-target species. A 50 m ×50 m grid was established in autumn 1999. Bait A recording of male Manx shearwater calls was played stations were made from 500 mm lengths of corrugated down 20% of burrows in each sub-area during the main plastic drainage pipe staked into position using wire. incubation period unless the sub-area contained fewer than A total of 1,260 stations were placed on the main island 50 burrows up to 2016 and then duetting male and female and offshore stacks. The poisoning operation ran from 11 calls were used for 2016 (Perkins, et al., 2017). In those January 2000 to 10 March 2000. Two 24 g blocks of cereal- cases, the recording was played in all burrows. Recordings based rodenticide bait (Neosorexa™, active ingredient were played at natural volumes (‘normal’ Manx shearwater 0.005% difenacoum, manufactured by Sorex Ltd) were call volumes as heard from the burrows that were set ‘by placed in each station on the main island and ten blocks ear’ before 2016 and by a decibel reader in 2016) within 30 on the offshore stack throughout the poisoning programme cm of the burrow entrance for up to 25 seconds. Playback and replaced as required when eaten by rats, non-target of calls was carried out in the day and responses, or lack species and/or damaged by weather. thereof, were recorded. Playback was undertaken between the end of May and mid-June at a time when all eggs laid The stations on the main island were checked daily, should be being incubated by one adult (Brooke, 1990). but stations on the offshore stacks were checked when sea conditions allowed. Bait take was recorded by bait station Between 2013 and 2016 nearly 100 artificial nest number and the species believed to have consumed or boxes were established on the island. These burrows are removed the bait. the same design as those developed in New Zealand for fluttering shearwaters (Puffinus gavia) by Bell (1995) and From 1 March to 15 March 2000, monitoring stations recommended for burrow-nesting petrel and shearwater were established around the island next to and in-between species (Gummer, et al., 2014). These artificial burrows the bait stations. Chew sticks, chocolate blocks and small were put in place to provide easily accessible study pieces of candle were used. Sand and mud areas on the burrows for tracking studies and to determine productivity island were checked for rat foot prints and burrows and and population parameters such as survival and recruitment rat runs were checked for fresh activity. All monitoring (Morgan, 2012; Kirk, et al., 2013). points were individually numbered and any evidence of activity (i.e. teeth marks or foot prints) was recorded by European storm petrel breeding population survey station number and the species believed to have consumed or marked the monitoring item. Each monitoring site was Surveys of suitable storm petrel habitat (i.e. stone checked regularly, either separately or together with the walls, rock tumbles and scree) on Ramsey Island were poisoning bait station grid. Any rat and non-target species undertaken using playback in 2008, 2010, 2012 and 2016. sign found on detection devices was recorded. A recording of a male European storm petrel was played close to a suspected site and a reply listened for (Ratcliffe, Manx shearwater breeding population survey et al., 1998; Gilbert, et al., 1999; Mayhew, et al., 2000). Burrow entrances that had responses were mapped using Earlier surveys on Ramsey Island had shown that Manx GPS. shearwaters only occur in a narrow strip around the coast
540 Bell, et al.: Seabirds on Ramsey Island after rat eradication
RESULTS Brown rat eradication Bait acceptance was good, with rats accounting for 165 The distribution of Manx shearwaters remained largely kg of bait consumed. As the LD50 for a 250-g brown rat is unchanged between 1999 and 2007 censuses, but the 9 g and the mean (± SE) bait take by rats was 81.6 ± 0.7 g range spread between the 2007, 2012 and 2016 censuses (3.4 ± 0.02 blocks, range 0–30 blocks) the rat population (Fig. 4). There have also been significant increases in the on Ramsey Island was estimated to be between 1,850 and population within the distribution with new areas recorded 5,400 rats). The bait take pattern was typical of other rat in 2007, 2102 and 2016 that previously had no responses eradication operations; very high in the immediate five to recorded in 1999 (Fig. 4). ten days after original baiting and dropping to a relatively Burrow density is greatest along the west, north and low level 21 days after original baiting. Bait take dropped north-east coasts and on the hills (Fig. 4). Interestingly to zero by day 41 after the original baiting (Fig. 2). The rats in a section at the northern end of Ramsey Island there were widely distributed across the island, but the density was no response to the recordings despite a high number was not even, as shown by the distribution of bait take (Fig. of suitable burrows available for Manx shearwaters (n = 3). Rats were present in all coastal areas and in highest 2,247) in 1999 or 2007. This area showed a low level of numbers within the central and northern areas of the island. response in 2012 and higher in 2016. Rabbits interfered with the bait stations between days A prospecting pair of Manx shearwaters was recorded 12 and 26 of the operation, with a number of carcasses in one of the artificial burrows in 2015. Two pairs nested being collected. Bait stations were modified by halving the successfully in the artificial burrows in 2016 and seven entrance size to prevent access by rabbits and this greatly pairs were recorded incubating eggs in April 2017. reduced their interference levels. Carrion crows (Corvus corone), ravens (Corvus corax) and herring gulls (Larus argentatus) also interfered with the stations from day 25 after the birds had learnt to reach into the stations to get access to the bait. Eight crows and three raven carcasses were located but no herring gull deaths were recorded. The bait stations were further modified by extending the length from 500 mm to 750 mm which reduced crow, raven and gull interference to almost nil. Crows were observed working in pairs to remove wires; one pulling the wire out while the other stood on top of the station to hold it in place, to get access to the bait. Access to the bait by the bank voles could not be prevented and 30 dead voles were found. A small captive population was maintained during the eradication as a precaution and was released after the poison had been removed. Voles and vole sign on monitoring tools were recorded throughout the eradication. Monitoring for rat presence continued for two years after the end of the poisoning operation. No rats or sign were detected. The rat-free status for Ramsey Island was declared in March 2002. Manx shearwater breeding population survey The number of Manx shearwater burrows on Ramsey Island totalled 13,800 burrows in 1999, 14,970 burrows in 2007, 12,302 burrows in 2012 and 12,319 in 2016 (Humpridge & Bullock, 1999; Morgan & Morgan, 2008; Morgan & Morgan, 2013; Morgan & Morgan, 2017). The Manx shearwater breeding population size increased 3-fold and 5-fold, 8 and 17 years after the rat eradication respectively (Table 1).
Fig. 2 Bait take by rats during the brown rat (Rattus Fig. 3 Distribution of bait take during the brown rat (Rattus norvegicus) eradication on Ramsey Island, Wales, norvegicus) eradication on Ramsey Island, Wales, 1999/2000. 1999/2000.
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Table 1 The total number of burrows, response rate used, total number of burrows sampled using playback, total number of responses and total number of breeding pairs of Manx shearwater Puffinus puffinus on Ramsey Island between 1999 and 2016.
Year Total number Response Total number sampled Total number Total number of of burrows rate used using playback of responses breeding pairs 1999 13,800 0.409 2,760 74 905 2007 14,970 0.409 3,190 208 2,387 2012 12,302 0.4625 2,788 402 3,835 2016 12,319 0.845 2,860 941 4,796
Fig. 5 Location of European storm petrel breeding sites (in black circle) on Ramsey Island, Wales, 2016.
Bait station design was adapted to suit local conditions and local non-target species. Fig. 4 Distribution and density of Manx shearwaters (Puffinus puffinus) from the full surveys in 1999, 2007, The Ramsey Island operation was the first island- 2012 and 2016 on Ramsey Island, Wales. wide eradication that used a rodenticide containing the active ingredient difenacoum proving that this rodenticide European storm petrel breeding population survey could be used to successfully target all brown rats on an island. This toxin has since been used on a number of The first storm petrel breeding burrows were detected island eradication operations in the UK and around the in 2008 (4 pairs). By 2016, the numbers had increased to world (Howald, et al., 2007; Jones, et al., 2008). Since the 12 breeding burrows (Table 2). eradication of rats, there have been biosecurity protocols put in place to prevent a re-incursion on Ramsey Island and DISCUSSION to outline how to respond if rats are ever detected on the island. It is important that these measures are maintained The Ramsey Island brown rat eradication was one of the indefinitely. first eradications undertaken by the RSPB and a number of important lessons were learnt that helped with the planning Ramsey Island has seen dramatic changes since and implementation of later eradications on Lundy Island, the removal of brown rats, not least the increase in the St Agnes and Gugh, Isles of Scilly and the Shiant Isles. distribution and density of Manx shearwaters. The number of Manx shearwaters has multiplied by five times between
542 Bell, et al.: Seabirds on Ramsey Island after rat eradication
Table 2 The number of burrows of European storm petrels current Ramsey Island storm petrel population. As the Hydrobates pelagicus on Ramsey Island between 2008 population increases, an island-specific correction factor and 2016. will be calculated for Ramsey Island and used to estimate the population size in the future. Currently, the minimum Year Number of breeding burrows estimate is used (i.e. the known response to taped calls). 2008 4 However, the basic playback-response method is 2009 Not surveyed widely used, standardised and is comparable between years and across sites. It is also a low-impact method, completed 2010 6 during the day, and provides spatial information on 2011 Not surveyed breeding burrows. The storm petrel population on Ramsey Island is likely to increase into a range of available habitat 2012 5 including drystone walls, rabbit burrows and rock tumbles. 2013 5 The success on Ramsey Island provided valuable 2014 5 information and techniques for later eradication operations in the UK, particularly those with important non-target 2015 8 species. It also showed that ground-based eradication 2016 12 techniques developed in New Zealand could be adapted and used on islands in the UK, and Ramsey Island serves as a good example of the significant long-term benefits that 1998 and 2016 to almost 5,000 breeding pairs, representing can be achieved through short-term investment. a 560% increase. This proves that the brown rats were having a significant role in suppressing the number of ACKNOWLEDGEMENTS breeding pairs on the island and their range across the island and provides more evidence that invasive rats have This project was carried out with funding from RSPB. significant impacts on seabird populations on islands Noesorexa™ bait was donated by Sorex Ltd. We would like (Atkinson & Moors, 1984; Atkinson, 1985; Towns, et al., to thank all the volunteers who helped with all stages of the 2006; Jones, et al., 2008). A similar pattern was observed eradication and subsequent Manx shearwater and European on Lundy Island following the rat eradication operation in storm petrel monitoring. Kelvin Floyd (WMIL) provided 2004 (Brown, et al., 2011). It is suspected that although Figs 1 and 3. Two anonymous referees commented on this increased productivity will have occurred on Ramsey paper. 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